Every type of cable medium, such as a twisted-wire pair used in telephone systems or a coaxial cable used in television (TV) systems, exhibits a frequency-dependent transfer function. The transfer function is one fundamental way to characterize the medium and may be advantageously used to determine the signal at the output of the cable for a given signal applied to the input of the cable.
A cable is a non-ideal transmission medium inasmuch as the cable introduces distortion into a signal propagating therethrough. This distortion is effected by amplitude and phase variations of the transfer function over the frequency range of the input signal. For example, for a rectangularly-shaped pulse signal, these variations result in a "smearing" or spreading of the pulse in time as well as a reduction in the peak of the pulse. An ideal but virtually non-realizable transfer function has a uniform or flat am characteristic and a linear phase characteristic over, at least, the frequency range of the input signal. With such a transfer characteristic, the output signal is a delayed version of the input signal differing only in the peak level.
Conventionally, in telephone systems, to correct for a non-ideal cable transfer function, a complex network using many components to compensate for standard lengths of cable (e.g., half-mile, one mile) was developed. The correction took the form of smoothing the amplitude characteristic and linearizing the phase characteristic so as to approximate the ideal transfer function characteristic. Since the complex network introduced additional loss or amplitude attenuation into the cascade connection of the cable and correction network, the network usually included a gain device. When the cable length was non-standard, a so-called "buildout" network was used in combination with the complex network. The buildout network included numerous circuit elements, some of which can be adjusted or set, to transform the odd length to a standard length for standard compensation. Since the buildout network typically was passive, it increased the actual electrical length of the cable to a longer standard length. Also, because of the limited number of settings for the buildout network, precise compensation was often not achieved, thereby resulting in only a partial compensation.
With respect to TV systems, some TV cameras are provided with built-in circuits to compensate for common cable lengths. If the actual cable length does not match that corresponding to one of the built-in compensation circuits, substantial signal distortion may occur. Other ways to overcome the distortion caused by cable lengths include using costly low-loss cables, microwave links or fiber optic cables. It is also possible to design a custom network comprising active and passive components to compensate for the exact cable length for a given installation. This approach is very expensive and requires skilled technicians to design and implement the custom network.
In the prior art of telephony, it is also known to provide an automatic line equalizer which adapts to the particular length and gauge of cable to which it is connected. Such an equalizer is disclosed in U.S. Pat. No. 3,568,100 (issued to R. A. Tarbox on Mar. 2, 1971 and hereinafter referred to as the '100 Tarbox patent). This equalizer utilizes an electronically controlled gain circuit responsive to the cable characteristics of the attached cable to adjust the frequency shaping of the gain, thereby providing the necessary frequency shaping for any length or gauge cable coupled to the equalizer. The equalizer has an embedded variable impedance network having only a single pole which is movable in the frequency domain in response to the automatic measurements. This equalizer provides an approximately correct compensation for only the high frequency range of the various cables and lengths in the telephone plant. The arrangement is most suitable for high frequency pulse transmission systems wherein the energy of the cable signal is localized to this high frequency range. The presumption with the circuit disclosed in the '100 Tarbox patent is that low and mid frequency losses are basically linear, but such a presumption is inadequate for systems having significant energy in the lower bands because of the inherently nonlinear low and mid frequency characteristics of cable.
Representative of more recent compensation techniques is the equalization circuit disclosed in U.S. Pat. No. 4,273,963 (issued to H. Seidel on June 16, 1981 and hereinafter referred to as the '963 Seidel patent).
Here, an active network is disclosed which corrects for varying cable lengths in a digital transmission system, that is, a system deploying rectangularly-shaped pulses to convey the non-zero signal or data state. The network disclosed in this patent requires an adjustable amplifying device having a balanced output including both plus and minus output terminals, as well as two frequency dependent paths coupled to the amplifying device, one to each amplifier output terminal. The compensation is a very non-linear function of the amplifier gain and typically requires a technician with great skill to perform cable measurements and then to adjust the equalization network for proper compensation.
Therefore, a need exists in the art for a cable compensation circuit, particularly suited for use with TV cables, that can properly compensate for substantially any normal length of cable, not just pre-selected "common" lengths, and is relatively simple to use and/or adjust and is inexpensive.